package com.learning.exam.util;

/**
 * 发号器
* Twitter的SnowFlake算法,使用SnowFlake算法生成一个整数，然后转化为62进制变成一个短地址URL
* @author beyond  https://github.com/beyondfengyu/SnowFlake
* @author xuliugen
* @date 2018/04/23
*/
public class SnowFlake {
    private SnowFlake(){

    }
    public static SnowFlakeInstance getInstance(){
        return SnowFlakeInstance.INSTANCE23;
    }
    public static SnowFlakeInstance getInstance(long dataCenterId, long machineId ){
        return new SnowFlakeInstance(dataCenterId,machineId);
    }
    public static class SnowFlakeInstance{
        /**
         * 起始的时间戳
         */
        private final static long START_TIMESTAMP = 1480166465631L;
        /**
         * 每一部分占用的位数
         */
        private final static long SEQUENCE_BIT = 12;   //序列号占用的位数
        private final static long MACHINE_BIT = 5;     //机器标识占用的位数
        private final static long DATA_CENTER_BIT = 5; //数据中心占用的位数
        /**
         * 每一部分的最大值
         */
        private final static long MAX_SEQUENCE = -1L ^ (-1L << SEQUENCE_BIT);
        private final static long MAX_MACHINE_NUM = -1L ^ (-1L << MACHINE_BIT);
        private final static long MAX_DATA_CENTER_NUM = -1L ^ (-1L << DATA_CENTER_BIT);
        /**
         * 每一部分向左的位移
         */
        private final static long MACHINE_LEFT = SEQUENCE_BIT;
        private final static long DATA_CENTER_LEFT = SEQUENCE_BIT + MACHINE_BIT;
        private final static long TIMESTAMP_LEFT = DATA_CENTER_LEFT + DATA_CENTER_BIT;
        private long dataCenterId;  //数据中心
        private long machineId;     //机器标识
        private long sequence = 0L; //序列号
        private long lastTimeStamp = -1L;  //上一次时间戳

        static final SnowFlakeInstance INSTANCE23 = new SnowFlakeInstance(2,3);
        /**
         * 根据指定的数据中心ID和机器标志ID生成指定的序列号
         * @param dataCenterId 数据中心ID
         * @param machineId    机器标志ID
         */
        public SnowFlakeInstance(long dataCenterId, long machineId) {
            if (dataCenterId > MAX_DATA_CENTER_NUM || dataCenterId < 0) {
                throw new IllegalArgumentException("DtaCenterId can't be greater than MAX_DATA_CENTER_NUM or less than 0！");
            }
            if (machineId > MAX_MACHINE_NUM || machineId < 0) {
                throw new IllegalArgumentException("MachineId can't be greater than MAX_MACHINE_NUM or less than 0！");
            }
            this.dataCenterId = dataCenterId;
            this.machineId = machineId;
        }

        private long getNextMill() {
            long mill = getNewTimeStamp();
            while (mill <= lastTimeStamp) {
                mill = getNewTimeStamp();
            }
            return mill;
        }

        private long getNewTimeStamp() {
            return System.currentTimeMillis();
        }
        /**
         * 产生下一个ID
         * @return
         */
        public synchronized String nextId() {
            long currTimeStamp = getNewTimeStamp();
            if (currTimeStamp < lastTimeStamp) {
                throw new RuntimeException("Clock moved backwards.  Refusing to generate id");
            }
            if (currTimeStamp == lastTimeStamp) {
                //相同毫秒内，序列号自增
                sequence = (sequence + 1) & MAX_SEQUENCE;
                //同一毫秒的序列数已经达到最大
                if (sequence == 0L) {
                    currTimeStamp = getNextMill();
                }
            } else {
                //不同毫秒内，序列号置为0
                sequence = 0L;
            }

            lastTimeStamp = currTimeStamp;

            long nextId = (currTimeStamp - START_TIMESTAMP) << TIMESTAMP_LEFT //时间戳部分
                    | dataCenterId << DATA_CENTER_LEFT       //数据中心部分
                    | machineId << MACHINE_LEFT             //机器标识部分
                    | sequence;//序列号部分
            return Long.toString(nextId);
        }
    }
}